A Friday night and I am sitting in what feels like a dying restaurant. It's not in intensive care yet. Think more high-dependency unit, its owners furiously studying the weekly figures for a settled pulse.

I am in Léon de Bruxelles, the only British outpost of the 120-year-old Belgian mussels-and-chips chain, which has dozens of branches across its home country and France. It's more than three-quarters empty. This is remarkable. Nothing around London's Cambridge Circus is empty, not on a Friday night. Garfunkel's is full. Café Rouge is full. Despite my very best efforts, the Angus Steakhouse, where they torture steaks nightly, is full.

But not Léon de Bruxelles. You can almost hear the wind blowing between the tables. The transparent laminate covering on the wipe-down mussel-shaped menus is curling off. Then again, why replace them if no one is coming? It can't claim a location problem. It occupies a huge site opposite London's Palace Theatre, is gilded in green bloody neon. Not finding it would require talent. And yet it's all shiny tables and empty seats.

I do find this odd. Mussels in the shell are one of the most gloriously compelling eating experiences there is. They turn the modern eater into ancient hunter-gatherer, roaming the tundra of shiny black shell in search of dinner. Plus, they are a brilliant leveller. However uptight you are, however much of a clean freak, it's impossible to eat them with cutlery. It's a sleeves-up, elbows-on-the-table, get-stuck-in job. Like the flick and pick of the pistachio nut from its shell, the very process of mussel eating is meditative. You zone out and stare deep into the steamy bowl until your eyeballs fog.

Or at least I do. That's why I wanted to go. Long ago I was quite the fan of Belgo, the London-based mussels-and-chips chain where the waiters were all forced to dress like Belgian monks, poor sods. It was serious man-on-mussel action. In 1996 Belgo Centraal, a cavernous and gloomy industrial basement space in Covent Garden which finally brought the aesthetic of a BDSM bar to eating out, was named London restaurant of the year. We were young then. They attempted a brand roll-out, but it didn't take. There are just four left.

And now there's this place which opened last year and which on a Friday night has many staff and few customers. The staff isn't the problem. They are friendly and engaged and have clearly read every page of the training manual, probably a few times. Our waiter almost sounds convincing when he declares their beer cocktails to be "very special". The thing is, special ain't the same as good. I knew a chap who could do something "special" with a lighter and his bodily methane. I wasn't paying for that either. I stick with a good wheat beer.

Part of the problem is the room, which is huge and sterile, with zany things scribbled on brightly coloured boards, the design equivalent of Timmy Mallett's glasses. On closer examination these turn out to be dish names. The room looks like the canteen of a direct marketing company which has tried to inspire the workforce by giving itself a half-arsed makeover. My wife looks around and declares the narcoleptic strawberry blush of the Angus Steakhouse down the road more welcoming. I know how to show a woman a good time.

On one wall there is a photograph of the Manneken Pis; on the other, the silhouette of Tintin. Of course these are not the only famous Belgians. There's a whole board listing "Les Belges Du Monde" –about 100 names in total, of which we recognise Jacques Brel, René Magritte, Brueghel and Johnny Hallyday. There's someone listed as Cockrent, which sounds like the name a male porn star would abandon for destroying all the mystique. On closer examination we identify a full stop between the C and O. We look forward to acquainting ourselves with the work of journalist Christina Ockrent.

On a high: waffle with ice cream. Photograph: Katherine Rose for the Observer

We have time to brood on all this because of the aching gaps between courses. There's nothing better guaranteed to make service run slow than a lack of people to serve. If you want to eat quickly, go to a busy restaurant. We watch those who arrived after us being served before us. The food, when it deigns to appear, has OKs and kill-me-now lows. A starter of warm smoked eel, with mustard-smeared toast, is all rich fish oils and kick. It's an expression of the Lowlands, the sort of thing you would eat to ward off chilly fogs. A plate of their charcuterie is a reasonable selection served, predictably, too cold. We let them flog us some slices of dead horse. It is smoked, a little sweet, and rather cloying.

And finally, the mussels. They needed to be good. Being a mussel restaurant that can't do good mussels is like being a cardinal who's crap at praying, or a slaughterman who can't stand the sight of blood. Léon de Bruxelles is all these things and far less. The meat inside the shells is small and shrivelled and dry; each shell contains what looks like the retracted scrotum of a hairless cat. They appear to have been left to steam for too long. Those with Dijon mustard are vinegary. I order the Madras mussels, because it's my stupid job to do so. It's exactly as you would expect Indian food to be were it cooked by Belgians. It smells of old curry-flavoured pot noodle; the flavour is not dissimilar. Eating these mussels is not meditative or compelling. It's just disappointing.

The unlimited chips come in deep ceramic pots and are crisp at the top and damp at the bottom where they have steamed in their own heat. Each mussel pot costs a fearsome £14.90.

We finish on a high with a freshly made waffle, a crisp puff of malty wonder with whipped vanilla cream, ice cream and maple syrup. I could say I'd come back for that waffle alone, but we all know I wouldn't.

Obviously our meal at Léon de Bruxelles isn't great. That goes without saying. But what really matters is that it's also terribly, terribly sad. That is a failure of a much deeper kind.

News bites

For more reliable mussel action head to Crab Shakk in Glasgow. This tight snug of a restaurant was the brainchild of an architectural practice, and looks like it: there's a lot of rough-hewn wood, a glass-topped bar and a serious bit of metal staircase. It's a good setting for proper seafood – piles of langoustine served cold or grilled, cracked crab claws, and, for £10.95, a big bowl of mussels. Crab Shakk, 1114 Argyle Street, Glasgow (0141 334 6127; crabshakk.com).

The words "purpose-built humidification" and "controlled moisture cabinet" are hardly the stuff of poetry, unless applied to beef. Lancashire butcher-turned-restaurateur Kevin Birkins has invested in major new kit to dry-age Bowland beef on the bone for 35 days, to be served at both the Fence Gate Inn near Burnley and the Eagle at Barrow near Clitheroe. Steak fetishists take note. fencegate.co.uk and theeagleatbarrow.co.uk/brasserie

Wonderful Soho ramen bar Tonkotsu is about to open a sibling in Hackney, but as yet doesn't have a drinks licence. As a result they're staging a bring-your-own booze event on 28 September, £30 for three courses. Visit tickettext.co.uk and search for Tonkotsu (tonkotsu.co.uk)